The present invention relates to a process and apparatus for forming a tube from a flat web. More particularly, the invention relates to the production of such a tube having a uniformity in circumference.
In preparing a tube from a flat web, folding aids in the form of forming shoulders or forming heads (German Offenlegungsschrift No. 3,013,320) are conventionally used. By this process, the web is bent and folded, so that the two edges of the web approach one another and the edge zones overlap. A tube with a longitudinal seam is formed by gluing the mutually overlapping edge zones. It is also possible to join the two edge zones to one another by means of a film strip, in which case the edge zones do not necessarily have to overlap. These known folding aids have the disadvantage that it is not possible to control the width of the overlap region, and thus the circumference of the formed tube, with particular accuracy. Moreover, in the case of forming shoulders, the web material is subjected to considerable mechanical stress, resulting from the large friction forces produced as the web is deflected on the collar edge of the forming shoulder over a short path.
According to U.S. Pat. No. 2,148,884, a mandrel arranged in the interior of the formed tube is used for support, while a roller engages the outside of the tube. These folding aids do not ensure exact guiding of the overlapping edge zones of the web, so as before, there is produced a tube of varying circumference.
The forming device disclosed in U.S. Pat. No. 2,856,742 comprises two mutually opposite forming parts, each externally contacting an edge zone of the web. The two forming parts are arranged at a mutual distance from one another in the direction of the forward movement of the tube. This known forming device is unsuitable for those webs which already carry an adhesive on one edge zone, before the tube is formed. The mutually overlapping edge zones are separated from one another only in the region between the two forming parts, so that there is a risk of the edge zones already being bonded to one another before they pass the first forming part.